The present invention relates to a novel gas turbine nozzle. Generally, a gas turbine nozzle has a construction as exemplarily shown in FIG. 1 and is produced by a precision casting. A typical example of materials of this gas turbine nozzle is a Co-base heat-resistant superalloy or Ni-base heat-resistant superalloy. The term "heat-resistant superalloy" will be abridged as "superalloy", hereinafter.
The Co-base superalloy exhibits a superior high temperature corrosion resistance at temperatures below 1000.degree. C., but suffers an inferior high temperature oxidation resistance at temperatures above 1000.degree. C. In addition, this superalloy has an inferior high temperature ductility and tends to become brittle to generate cracks by an application of an external force such as thermal stress. When a diffusion coating of Al is applied, .delta. phase of Co-Al compound is formed thereby causing an embrittlement. Furthermore, this superalloy exhibits an inferior weldability.
There are two types of Ni-base superalloy: namely, .gamma.' phase strenghtening type superalloy making use of the precipitation of Ni.sub.3 (A1,Ti) which constitutes the .gamma.' phase, and a carbide strenghtening type superalloy. The Ni-base superalloy of .gamma.' phase strengthening type, on one hand, exhibits a superior high temperature oxidation resistance at temperatures above 1000.degree. C. but, on the other hand, suffers an inferior high temperature corrosion resistance at temperatures below 1000.degree. C. due to a small Cr content. In addition, this superalloy contains Ti and Al in excess of solid solution limit and is strengthened by .gamma.' phase, so that this superalloy exhibits a large high temperature strength, but the thermal fatigue resistance, which is an important property for the material forming a gas turbine nozzle, is lower than that of the Co-base superalloy. The Ni-base superalloy of .gamma.' phase strengthening type, therefore, cannot be used suitably as the material forming a mechanical part which is subjected to repetitional heat cycles. It is to be pointed out also that the melt of this superalloy has to be made by vacuum melting, because of its large Ti and Al contents. This superalloy, therefore, is not suitable for use as the material forming a gas turbine nozzle having a large size.
The Ni-base superalloy of another type, i.e. the carbide strenghtening type, has superior high temperature strength, ductility, creep rupture strength, thermal fatigue resistance (resistance to thermal shock) and high temperature corrosion resistance at temperatures around 982.degree. C. at which the gas turbine nozzles are used. In addition, this superalloy can be produced easily by melting in air atmosphere. On the other hand, however, this superalloy exhibits only small ductility and, moreover, a poor thermal fatigue resistance (resistance to thermal shock) which is an important factor for gas turbine nozzle material, at temperatures around 800.degree. C. to which the blades are heated in general purpose gas turbines that operate at gas temperatures higher than 1000.degree. C. This fact is attributable to the presence of cellar continuous eutectic carbide in the grain boundary. The microstructure of carbide strengthening type Ni-base superalloy contains eutectic carbides crystallized in the grain boundary and secondary carbides precipitated mainly in the grains. A certain amount of eutectic carbides is effective in improving the creep rupture strength through suppressing the grain boundary sliding. It has been proved, however, that the presence of the coarse eutectic carbides in cellar continuous form in the grain boundary promotes the propagation and development of cracking due to the stress concentration to the brittle eutectic carbides by application of thermal fatigue (thermal shock), particularly when the material is subjected to a high temperature and repetitional heat cycles of heating and rapid cooling as in the case of gas turbine nozzles. It has been proved also that such eutectic carbides are thermally stable and are not changed substantially by ordinary heat treatment.
Examples of gas turbine nozzles made of Ni-base superalloy are disclosed in the specification of the U.S. Pat. No. 4,283,234. This Ni-base superalloy, however, has a low cobalt content, so that it is inferior in creep rupture strength and thermal fatigue resistance.